1. Field of the Invention
The present invention relates to a cryostat with a pipe which connects a liquid gas storage tank for liquid helium or the like to a normal temperature section.
2. Description of the Prior Art
A magnetic resonance imaging (MRI) device which uses a superconducting magnet for generating a static magnetic field utilizes a cryostat provided with a liquid gas storage tank for liquid helium or the like.
FIG.10 is a schematic cross-sectional view of a conventional cryostat. As shown in the drawing, a first stage radiation shield plate 2 and a second stage radiation shield plate 3 are arranged in a vacuum tank 1, and a GM (Gifford-McMahon) refrigerator unit 4 is connected to the radiation shield plates 2, 3.
A liquid helium tank 6 for storing liquid helium 5 is positioned inside the second stage radiation shield plate 3, and a superconductive magnet 7 is positioned so that it is immersed in the liquid helium 5 in the liquid helium tank 6. A pipe 8 which passes through the vacuum tank 1 and the first and second stage radiation shield plates 2, 3 to communicate with a helium recovery section (omitted from the drawing) of a normal temperature section, is connected to the liquid helium tank 6. The pipe 8 is cooled at a first stage anchor section 9 and a second stage anchor section 10 of the first and second stage radiation shield plates 2, 3 (for example, to about 40K to 45K at the first stage anchor section 9 and to about 5K to 9K at the second stage anchor section 10).
In addition, in order to avoid the penetration of heat into the liquid helium tank 6 through the pipe 8 from the helium recovery section (omitted from the drawing) of the normal temperature section as the result of convection, heat conduction, and radiation of the helium gas, a cylindrical member 12 made from fiber reinforced plastic (FRP) with a plurality of baffle plates 11 installed in the peripheral direction on the outer peripheral surface is inserted into the pipe 8, as shown, for example, in FIG. 11, or, an FRP rod 14 with a plurality of baffle plates 11 installed in the peripheral direction is inserted into the pipe 8, in place of the cylindrical member 12, as shown in FIG. 12.
In this manner, in the above-mentioned conventional cryostat, the cylindrical member 12 with a plurality of baffle plates 11 installed in the peripheral direction on the outer peripheral surface (see FIG. 11), or the FRP rod 14 with a plurality of baffle plates 11 installed in the peripheral direction (see FIG. 12), is inserted in the pipe 8 connecting the liquid helium tank 6 with the helium recovery section (omitted from the drawing).
As a result, in the case where these, the cylindrical member 12 and the FRP rod 14, are not inserted into the pipe 8, the amount of heat penetrating from the helium recovery section (omitted from the drawing) through the pipe 8 to the liquid helium tank 6 which is about 19 mW (at a temperature of the second stage anchor section of about 7K), can be dropped to about 9 mW (at a temperature of the second stage anchor section of about 7K).
With the above-mentioned conventional cryostat, the cylindrical member 12 with a plurality of baffle plates 11 installed in the peripheral direction on the outer peripheral surface (see FIG. 11) or the FRP rod 14 with a plurality of baffle plates 11 installed in the peripheral direction (see FIG. 12) is inserted into the pipe 8 to reduce the amount of heat penetrating from the normal temperature section of the helium recovery section (omitted from the drawing) to the ultra-low temperature section of the liquid helium tank 6.
However, with the above-mentioned type of conventional cryostat, the convection of the helium gas to the liquid helium tank 6 from the helium recovery section (omitted from the drawing) is an unregulated flow because the baffle plates 11 are positioned in multiple stages in the pipe 8.
It is therefore not possible to adequately cut off the convection to the liquid helium tank 6 from the helium recovery section (omitted from the drawing).
For this reason, the liquid helium 5 evaporates from the liquid helium tank 6, giving rise to the necessity of replenishing the liquid helium frequently. Maintenance costs are therefore high.